It is well known within the art that one of the acoustic parameters that affects the perceived sound quality in a listening room, for instance a concert hall or auditorium, is the reverberation time of the room. However, the optimal reverberation time differs for various kinds of music and for speech, recommended reverberation times for rooms in which classical music is to be performed thus being in the range of 1.5 seconds to 2.0 seconds, whereas rooms for performance of rhythmic music have recommended reverberation times in the range of 0.8 seconds to 1.0 seconds. Even shorter reverberation times may be beneficial for auditoriums in order to attain the best possible speech intelligibility. Furthermore, the reverberation time should ideally be almost the same throughout the relevant frequency range of the program material. Typically, however, the reverberation time tends to decrease as a function of frequency, e.g. due to higher sound absorption in air at high frequencies, increased sound absorption at the boundaries of the room at higher frequencies as well as due to the presence of people in the room. Thus, low-frequency reverberation often tends to be too high compared with high-frequency reverberation, which may lead to an unacceptably “boomy” reproduction of sounds in the room, a loss of perceived details of the music and even to a deterioration of speech intelligibility. FIG. 1 shows measured reverberation time as a function of frequency of seven different rooms that may be used for live performances or reproduction of music. The figure shows an average reverberation time T30 above 500 Hz of approximately 1 second, whereas the average at low frequencies increases to approximately 1.5 seconds at 63 Hz. It also appears from FIG. 1 that large variations of reverberation time exist between the different rooms.
In view of the above there often exists a need for means for altering the reverberation time of a given room in a desired manner, and especially at low frequencies a selective reduction of reverberation time would be beneficial.
Devices for altering the reverberation time of a listening room are known within the art. Some of these are predominantly effective at higher frequencies, where the reverberation time may be reduced simply by providing thin layers of an acoustic absorptive material—a thin layer of mineral wool covered by a protective screen for instance—on chosen boundaries of the room. Selective reduction of reverberation time at low frequencies is somewhat more difficult to implement, although a number of actual implementations have been successfully applied for many years. Three different implementations of reduction of reverberation time at low frequencies—which to some extent also functions at higher frequencies—should be mentioned:    1. A sufficiently thick panel of an acoustic absorptive (porous) material will lead to sound absorption at low frequencies (as well as at higher frequencies) provided the thickness of the panel is sufficiently large compared with the wavelength of the sound at the lowest frequency at which an effective reduction of reverberation time is required. Example of materials applicable for such panels are glass fibre, mineral wool and sintered metals. Such panels may be mounted directly on a boundary or separated from the boundary by an air space, which will improve performance at low frequencies. The panels may also be hung from the ceiling thus giving access to the panel from both sides. Apart from the required thickness, which may exceed one meter if significant low-frequency absorption of acoustic energy is to be expected, such panels will not selectively absorb sound at low frequencies but rather exhibit a sound absorption as a function of frequency which will be fairly constant above a given lower limiting frequency—determined among other things by the thickness of the panel and the acoustic properties of the particular material being used—and decrease below this lower limiting frequency, thus not being able to provide selective low-frequency reduction of reverberation time as often required.    2. Low-frequency sound absorption can be attained within a limited bandwidth of for instance one octave around a given resonance frequency by the application of so-called panel absorbers or membrane absorbers, basically consisting of a rigid frame adapted for mounting on a wall or other boundary of a room. Over the frame and at a given distance from said wall or boundary there is provided a thin, flexible panel for instance of plywood, which is brought to vibrate driven by the sound field in the room. The mass and stiffness of the panel together with the compliance of the air volume defined by the frame, the panel and the boundary behind the panel will determine the resonance frequency of the absorber and the internal losses will determine the Q value of the resonator and hence its bandwidth. In order to increase absorption as well as to change the Q value of the absorber, acoustic damping material such as mineral wool may be introduced into the cavity within the frame. As the compliance of the air in the cavity depends on the volume of air in the absorber, the resonance frequency may be changed by changing the depth of the resonator and maintaining the circumferential dimensions of the frame. A deeper absorber thus provides a lower resonance frequency. A more rigorous description of these mechanisms will be given in the summary of the present invention    3. Low-frequency sound absorption can furthermore be attained using a so-called Helmholz resonator basically consisting of one or more passages or tubes of a given length and cross sectional area, these one or more passages representing an acoustic mass, where one longitudinal end of one or more passages is/are coupled to the sound field in the room and the other end is coupled to a cavity of a given volume representing an acoustic compliance essentially proportional with the volume of the cavity. The particular combination of mass and compliance determines the resonance frequency of the Helmholz resonator and the internal losses determine the Q value or effective bandwidth of the Helmholz resonator. At and around the resonance frequency, the input impedance of the resonator will be very low and the resonator will hence absorb sound energy from the surrounding sound field selectively in a frequency region around the resonance frequency. As in the case of the panel absorber, damping material such as mineral wool may be introduced in the Helmholz resonator to alter the Q value hereof. In practice Helmholz resonators are often of a form somewhat resembling the above mentioned panel resonators, where the thin, flexible panel have been replaced by a thicker, rigid panel provided with a pattern of passages through the panel. However, Helmholz resonators comprising a single passage or tube and a cavity have also been used for changing the reverberation time and/or suppression of undesired low-frequency room modes.Background Theory of Membrane Absorbers
A membrane absorber typically consists of a light plate in front of a closed cavity. Often the cavity is filled with a porous material, which provides damping for the system. When deriving the theoretical characteristic equations for a membrane absorber, the walls and back of the cavity are assumed to be rigid and the bending stiffness in the plate is assumed to be negligible compared to the stiffness of the air column in the cavity. The system is characterized by the mass per unit area of the plate, m, the depth of the cavity, d, and the internal losses of the system, ri, consisting of the losses due to the flow resistance of the porous material, internal losses in the plate and losses in the joints along the edges of the plate, ρ is the density of air or other gas in the cavity and c is the speed of sound.
The acoustic impedance of the system can be shown to be:
  Z  =            r      i        +          j      (                        ω          ⁢                                          ⁢          m                -                              ρ            ⁢                                                  ⁢                          c              2                                            ω            ⁢                                                  ⁢            d                              )      
The resonance frequency of the system is found when Im{Z}=0:
      f    0    =            c              2        ⁢        π              ⁢                  ρ        md            
This shows that the resonance frequency, where the absorption should be highest, is inversely proportional to the square root of both the mass of the membrane and the depth of the cavity. According to this theory, in order to obtain a maximum absorption at around 63 Hz, with a cavity depth of 0.2 m, the membrane must have a mass of about 5 kg/m2. But by pressurizing the cavity, the stiffness of the system grows and it may be possible to apply a less heavy material.
The impedance of the absorber can be tuned in order to maximize the absorption at the resonance frequency and the usable bandwidth of the absorber (half-power bandwidth, Br). If the impedance is too high, relative to the radiation resistance of the membrane, rs, the incident sound field will reflect off of the membrane and not be absorbed. If the impedance is too low, then the internal losses will be too small and not enough sound energy will be absorbed. The impedance ratio of the internal losses and the external radiation resistance can be expressed as:
  μ  =            r      i              r      s      
The maximum absorption coefficient and absorption bandwidth can then be written as:
            α      max        =                  4        ⁢        μ                              (                      1            +            μ                    )                2                                B        r                    f        0              =                  (                  1          +          μ                )            ⁢                                    ρ            ⁢                                                  ⁢            d                    m                    
Above it has been assumed that the absorbing device be of substantially the same depth d throughout the device. For many of the embodiments of the present invention described in the detailed description of the invention this will not be true, the depth d changing in a characteristic and predetermined manner over the surface of the absorbing device. In such embodiments it may still be possible to apply the above expressions to determine at least approximate values of resonance frequency, absorption coefficient and absorption bandwidth by insertion of an average value of the depth d of the device. Alternatively, the above expressions may be reformulated in terms of the actual air or gas volumes and the corresponding compliances as is known within the field of acoustics.
Measurement of Reverberation and Absorption Coefficients
The absorption coefficients of the test specimen can be calculated from the measured reverberation time of the empty reverberation chamber and the reverberation chamber with test specimen present as follows:
  α  =                    55.3        ⁢        V                    cS        S              ⁢          (                        1                      T            60            S                          -                  1                      T            60                              )      where V is the volume of the reverberation chamber, SS is the area of the test specimen, T60S is the reverberation time in the chamber with the specimen present and T60 is the reverberation time of the empty chamber.
The above prior art absorbers may attain very high absorption coefficients at and in the vicinity of the resonance frequency and absorption coefficients in the order of 0.9 may well be attained with such absorbers. Nevertheless such prior art absorbers suffer from a number of disadvantages, some of which are described in the following.
The sound absorption characteristics of the above prior art absorbers can not readily be altered once the absorber has been constructed. Specifically major changes of the absorption coefficient α and/or the resonance frequency can not be accomplished by minor modifications of a given absorber. Also the absorption coefficient can not be changed systematically in a simple manner, such changes comprising for instance a shift between a very high absorption coefficient and a very low absorption coefficient, i.e. essentially an on/off function of the absorber.
The above-mentioned absorbers are rather bulky structures that will be difficult—or occasionally even impossible—to remove from a given room once installed. They are to be regarded as fixed installations in the particular room and not installations that can readily be dismantled from a given room, transported to another room and used here. Even though dismantling and transport to another room may be possible, great costs would be incurred by the transport due to the bulky nature of such absorbers.
Even though acoustic absorbers of the above kind may not have to be transported to another room for application here, it might be desirable under some circumstances to apply a given number of absorbers in a room and under other circumstances a lesser number of the absorbers, or even no absorbers at all might be needed for instance dependent on the kind of musical performance planned for the room. Storage of a large number of rather bulky absorbers in-house could well be a problem in these cases.